1. Field of the invention
This invention relates to apparatus for forming and severing an extrudate of a flowable material into separate blanks of controlled volume. Such an apparatus is commonly known as a "preformer".
2. Background prior art
U.K. Patent Specification No. 1141033 describes and illustrates "Preformers" which have been used for the manufacture of rubber moulding blanks for compression moulding since 1966, when Barwell Engineering Limited first introduced an accurate means of extruding rubber through a die, and cutting it at the die face before swell could occur. In this case, accuracy was achieved by monitoring and controlling the volume of rubber being extruded, and this was made possible by using a hydraulic ram extruder. The limitations of this technology are:
(a) The machines have to be fed with pre-warmed rubber which is of reasonably even plasticity throughout and relatively air-free. This usually requires the use of a rubber mill, which is an expensive piece of machinery to buy and run.
(b) A ram extruder is a batch-type machine and hitherto could not be made to operate on a continuous basis.
(c) It is necessary to draw vacuum from the material in the extrusion barrel before commencing an extrusion operation.
Barwell also produced an extrusion apparatus intended to provide a continuous extrusion in which two hydraulic-ram extruders were connected through a switch-over valve to a common outlet conduit. There were difficulties with this arrangement in maintaining a uniform extrusion rate during changeover from one extruder to the other.
In later years, further developments were made by Barwell and others to meet the demand from the industry for a cold-fed machine, to eliminate the problems of pre-warming the rubber. These attempts involved the use of a cold-feed screw extruder to masticate the rubber and induce heat through friction as it passed through the barrel. The rubber was then fed into a conventional ram extruder for the preforming operation to take place. A further development was the use of a rotary encoder to monitor the movement of the ram, and thus control the operation of the knife to achieve the accuracy required. This is an alternative to using a fixed cutter speed and controlling the volume of the extrudate.
Although a section of the industry continues to want a fairly sophisticated preformer capable of being fed semi-automatically with cold rubber compound, the above two attempts made so far have not been wholly successful. The first is too complicated, difficult to clean and will only work with a limited range of rubber compounds. The second is too large and expensive to interest the majority of potential users.
German Patent Specification No. 1007053 discloses another approach in which an extruder comprises a pair of extruder cylinders which are fed alternately by a plasticizing screw extruder under the control of a changeover switch, the arrangement being such that when one cylinder is extruding, the other is being recharged. Whilst this arrangement enables a continuous flow of extrudate to be produced, there is no precise control over the flow rate of extrudate delivered since each extruder cylinder operates independently of the other.
A similar arrangement is disclosed in Belgian Patent Specification No. 568274 in which an injection process for manufacturing, profiled blanks from a plastics material is disclosed in which a basic material to be injected is continually plasticized and is introduced alternately into first and second dosage chambers in such a way that, whilst one dosage chamber is being filled, the material contained in the other dosage chamber is being dispensed into the injection mould cavity. The plastification apparatus comprises a endless screw for working and pushing forward the material to be injected, this screw being driven continuously and the dosage chambers comprise two hydraulic ram operated extrusion cylinders. Each extrusion cylinder is operated independently of the other and there is no provision for precise control of the volume flow rate of the extrudate. Other similar forms of extrusion apparatus are described and illustrated in British Patent Specification No. 604,241 and British Patent Specification No. 1,137,430.